The invention relates to a ceramic body of a dielectric material on the basis of barium titanate, which material comprises a donor material, and to a method of manufacturing such a ceramic body.
The invention also relates to a ceramic capacitor which is composed of electrodes between which a ceramic body is disposed.
The expression "dielectric material on the basis of barium titanate" is understood to include not only pure barium titanate BaTiO.sub.3 but also material in which Ba is partly substituted by Ca, Sr or Pb and/or in which Ti is partly substituted by Zr or Sn. These substitutions, which are customarily present in the form of impurities, do not have an adverse effect up to a quantity of several atom %. However, the dielectric constant is highest in barium titanate in which Ba and Ti are not substituted by elements having different valencies such as donors and acceptors, which is important for the manufacture of, for example, small-volume capacitors having a large capacitance.
The electric resistance of barium titanate depends on the temperature and the electric field strength. Pure barium titanate is unsuitable for use in a capacitor in which the layer thickness of the dielectric material is small, for example smaller than 15 .mu.m, because both at low and high temperatures (higher than 100.degree. C.) the electric resistance is insufficient. Partly due to the temperature dependence of the dielectric constant of barium titanate, donor materials and acceptor materials are added. Thus, Ti in BaTiO.sub.3 is partly substituted by, for example, Nb (donor) and/or Co (acceptor). Up to a quantity of approximately 0.3 atom % a substantial grain growth takes place when the ceramic material is sintered. The material behaves like a semiconductor having a resistance of 10 to 100 .OMEGA. cm. Such a material is suitable for use as a PTC resistance material. At larger quantities of the donor and acceptor materials, in excess of 1 or 2 atom %, only a limited grain growth takes place during sintering. In this case, the material has a high electric resistance, between 10.sup.10 and 10.sup.14 .OMEGA. cm, and is suitable for use as a dielectric material. The addition of the donor and acceptor material leads, however, to a decrease of the dielectric constant, which decrease is larger according as more materials are added. At room temperature, pure barium titanate may exhibit a dielectric constant in excess of 8000. The dielectric constant of BaTiO.sub.3 is maximal at a grain size between 0.7 and 1 .mu.m and decreases rapidly when the grain size increases.
In British Patent Specification GB 1,241,974 a description is given of the preparation of ceramic dielectric material on the basis of barium titanate, powders having a particle size from 0.5 to 2 .mu.m and a crystallite size from 0.3 to 0.5 .mu.m being used as the starting material. By sintering at temperatures from 1260.degree. to 1455.degree. C., ceramic bodies are manufactured having grain sizes from 2 to 20 .mu.m as a result of recrystallization. The dielectric constant of the material obtained is between 1840 and 2750. The material according to said British Patent Specification comprises relatively large quantities of crystal-growth inhibiting agents. Ca, Sr, Bi and Pb are substituted for Ba, and Nb, Sb, Zr, Ta and Sn are substituted for Ti, in quantities from 7 to 14%. The materials exhibit a suitable temperature dependence but they have a small dielectric constant. Owing to crystal-growth which despite the presence of growth-inhibiting agents still takes place to an undesirable extent, said materials still exhibit semiconducting properties at higher temperatures, as a result of which the electric resistance at high electric fields is insufficient for use in capacitors, in particular, in capacitors having a small distance between the electrodes.